Audio amplifier



1 s. BERNSTElN-BERVERY 2,947,947

AUDIO AMPLIFIER Filed April 7, 1959 2 s -s 1 WATTS 2 TUBES muzqmwm W210 E c N A D E P M S M H 0 I.M. SMALL SIGNALS l o o l 00o 432 Q Q Q QlIl 0 25m |0|||l-l|ll|ll-llll 0000 mm o zoiumzzoo l ma RATIO Oi SCREEN WINDINGS TURNS TO ANODE WINDING TURNS ZOFOMZZOQ mcomkwk INVENTOJR. SERGIO BERNSTElN-BERVERY ATTORNEY Filed April 7, 1959 2 Sheets-Sheet 2 INVEN TOR. SERGIO BERNSTEIN-BERVERY ATTORNEY United States Patent AUDIO AMPLIFIER Sergio Bernstein-Bervery, Tarrytown, N.Y., assignor to General Precision, Inc., a corporation of Delaware Filed Apr. 7, 1959, Ser. No. 804,814

4 Claims. (Cl. 330-117) The invention provides a circuit which is particularly suitable for use as the final or power output stage of an audio amplifier. It secures high power output together with low output impedance. It generates no even harmonics and has at least as low odd harmonic generation as other high-quality circuits. Intermodulation distortion is low at both high and low signal power levels. This circuit is particularly advantageous in permitting use of an output transformer which is not of high quality to secure the highest quality of signal transmission. The transformer employed may have relatively low coupling and high capacitance between its two primary windings without reducing the transmitted signal quality.

The circuit of the invention employs two similar tubes in push-pull. Any tube type having a screen grid may be employed in the push-pull stage. For example, these tubes may be tetrodes, pentodes, or beam power tubes, the latter being sometimes termed a beam tetrode or beam pentode. The two push-pull tubes are driven from a phase inverter by signals of opposite phase. The two tubes may be operated in class A, AB or B, but are advantageously operated in class AB or B for in the circuits of this invention the greater efficiency of such operation is secured without generation of load-switching transients. The two tubes are energized in parallel by direct current from the B-supply. Their signal outputs drive two primary windings of an output transformer. primary windings is tapped for connection to both screen grids but, alternatively, both screen grids may be connected toa single tertiary winding inductively coupled to the primary windings. A secondary winding is connected to drive a speaker.

In the conventional push-pull circuit operated other than class A, each tube works individually into one-half of the primary winding. The two halves of the primary winding carry signal currents in alternation, and not simultaneously, for at least part of the time. Since there is never 100% coupling between the two halves of the primary winding, transients are generated at the instant when the signal load is shifted from one to the other half. Additionally, any stray capacitance between windings of the output transformer causes the circuit loss to vary with frequency. As a result, an output transformer for the conventional push-pull circuit must be made with extreme care to secure maximum coupling with minimum leakage reactance and minimum interwinding capacitance.

Tht instant invention employs a modification of the conventional push-pull circuit permitting use of an output transformer having low coupling between two primary windings with associated high leakage reactance, and

without any special provisions to minimize interwinding capacitance.

7 When triodes are employed in a push-pull-class A output stage a very low level of small-signal intermodula- One of these tion distortion is secured together with low internal impedance. On the other hand, when tubes having screen grids are employed output power capability is at least doubled and large-signal intermodulation distortion is reduced. In a conventional push-pull stage the triode advantages are not secured when a screen grid circuit is employed and the screen grid circuit advantages are not found in the triode circuit. However, in the circuit of the present invention all of these advantages are secured at the same time. Although screen grid tubes are employed in class AB or class B operation, with their above advantages, the advantages usually associated only with triode operation are also secured while load-shifting transients are not generated.

p The purpose of this invention is to provide a tube amplifying stage employing a simply-designed transformer yet providing all of the advantages of the push-pull circuit operated with triodes together with all of the advantages of the push-pull circuit in class AB orB operation employing tubes containing screen grids.

A further understanding of this invention may be secured from the following detailed description together with the associated drawings, in which:

Figure 1 is a schematic circuit diagram of an embodi: ment of the invention.

Figure 2 depicts a portion of a circuit similar to that of Figure 1 containing alternative connections for triode and tetrode operation of one tube.

Figure 3 depicts graphs illustrating the operation of the invention.

Figures 4 and 5 depict alternative embodiments of the invention.

Referring now to Fig. 1, a transformer 11 suitable for audible frequency currents has a primary winding 12 and two identical secondary windings 13 and 14 with relative polarizations indicated by the dots thereon. A pair of identical electronic tubes 16 and 17 are of a beam power type such as, for example, types 6L6, 807, 5881 and 6550. The dot-marked end of secondary winding 13 is coupled through a capacitor 18 to the control grid 19 of tube 16 and the other end of this winding is connected to cathode 21. The dot-marked end of winding 14 is connected to the cathode 2'2 of tube 17 and the other end of this winding is coupled through a capacitor 23 to control grid 24. The control grids 19 and 24 are connected to a negative terminal 26 through resistors 27 and 28. The anode 29 of tube 16 is connected to the positive terminal 31 of a source 32 of direct current, while the other terminal 33 thereof is connected to one end of one primary winding 34 of an output transformer 36. The other end of primary winding 34 is connected to cathode 21. The cathode 22 of tube 17 is connected to the negative termi nal 37 of a second source 38 of direct current having its positive terminal 39 connected to one end of a second primary winding 41 of the output transformer 36. The other end of winding 41 is connected to anode 42 of tube 17. a

The polarization of primary windings 34 and 41 are defined by a dot at one end of each winding symbol. The dot-marked ends are connected respectively to cathode 21 and anode 42. These dots indicate primary specialized design demanded by most high quality am- P -ifier circuits.

The primary winding 41 is provided with a center tap 46. This tap 46 is connected to the screen grid 47 of tube 17 and also to the screen grid 48 of tube 16.

The circuit terminals 33 and 37 should both be connected to a common reference potential terminal, preferably ground. A large capacitor 49 is connected between circuit terminals 33 and 39 and a similar large capacitor 51 couples cathode 21 and anode 42. These capacitors must be so large as to offer negligible impedance to currents of the lowest frequency for which the circuit is designed. Neither of these capacitors is essential to operation of the circuit, but they improve operation at both the high and low ends of the frequency range because a practical transformer 36 is necessarily less than ideal in coupling and in distributed capacitance. It follows that when the capacitors are used the two primary windings are clearly in parallel for signal currents at all time.

The batteries 32 and 38 are conventionally assumed to be shunted by large capacitors and to have low resistances so that their impedances to signal currents are negligibly low. They also are assumed to be equal in their potentials. In Fig. 1 and in all of the other figures a single battery of the same voltage may be substituted for the two batteries 32 and 38 without affecting practical operation, but two batteries are depicted in Fig. 1 because the mode of operation of the circuit is thereby more easily grasped.-

In class B operation of the circuit of Fig. 1, the quiescent biasing potential at terminal 26 is made high enough just to cut off anode current in both tubes 16 and 17 in the absence of an input signal. Since the input secondary windings 13 and 14 are connected to apply reversed-phased signals to the tubes 16 and 17, when the input signal makes grid 19 more positive relative to its cathode 2-1 the grid 24 is made more negative, and vice versa. Thus the tubes 16 and 17 are driven in opposed phases. The anode direct-current energizations of tubes 16 and 17 are in, parallel since their cathodes 21 and 22 are connected to the same potential point and their anodes 29 and 42 are at the same po'sitive potential if batteries 32 and 38 have the same potential. The screen grids 48 and 47 are also energized by direct current in parallel, for they are fed from the same point 46. When the input signal phase applied to tube -17 control grid 24 is positive, this tube conducts and the instantaneous current flow in its anode circuit may be traced from junction 39 through primary winding 41 to anode 42. Additionally, a second parallel path exists having substantially the same impedance extending from junction 39 through capacitor 49, primary winding 34 and through capacitor 51 to anode 42. In a similar manner, when tube 16 is conductive signal current flows from cathode 21 through primary winding 34, junction 33 and battery 32 to anode 29. Signal current also flows through the parallel path from cathode 21 through capacitor 51, primary winding 41, capacitor 49 to junction 33 and anode 29. Thus during all parts of the signal cycle the two primary windings 41 and 34 are in parallel for signal currents.

It is to be noted that since substantially equal signal currents flow simultaneously in the same phase through both primary windings, there is no tendency to generate a switching transien due to transfer of load from one winding to the other, and there is no requirement for close coupling between these two windings to minimize such transient, as is required in conventional. push-pull output transformers. Further the terminals '52 and 53 of primary windings 34 and 41 are always at the same signal potential, and the terminals 54 and 56 are always at the same signal potential. Therefore. all other: similar points throughout these windings are also always at the same signal potential. It follows; that capacitances between these two windings, if between similar points,

4 have no effect on operation because a signal potential difference never exists across them.

With respect to switching transients in the midfrequency range, when the primary windings 41 and 34 are closely coupled the capacitor 51 may be omitted with negligible effect on quality of transmission. However, at very low and very high audio frequencies when insufficient coupling exists between the primary windings,

resulting in some leakage reactance, the capacitor 51 insures that the 'coil' terminals 51 and 52 are at the same signal potential at all times and transient disturbances are avoided.

In Fig. 2- a portion of the circuit of Fig. l is reproduced but with a switch 57 inserted. When switch 57 engages its contact 58 the screen grid 47 of tube 17 is connected to its anode 42 and that tube behaves as a triode. It has low internal anode-cathode impedance, low output power capability, low low-level intermodulatio'n distortion and high high-level distortion. When switch 57 engages its contact 59 the tube behaves like a tube having a screen grid, in this case as a beam power tube. It has high anode-cathode impedance, high output power capability, high low-level distortion and low high-level distortion. If, however, the grid 47 should be connected to an intermediate point on winding 41, behavior in operation is intermediate between triode behavior and screen grid tube behavior. It is possible to pick a point of connection which will retain most of the advantages of both triode and screen grid operation, this point of connection being different for different types of tubes. This fact is clearly shown by Fig. 3, in which the variations in these four characteristics of. operation with variation of tap position are shown for the type 6L6 tube. Curve 61 shows the variation of final stage anode circuit impedance as measured at the 16-ohm terminals of the secondary winding 43 of the step-down output transformer 36. Curve 62 shows the manner in which the maximum undistorted output signal power obtainable from the stage varies with the position of the screen grid tap. Curve 63 shows the variation in intermodulation distortion for large-signal input and curve 64 shows the same for small-signal input. When the number of turns in the portio'n of the primary winding serving the screen grid is between 40% and 50% of the whole, the greater part ofv the tetrode connection advantages are retained, while at the same time most of.

the triode connection advantages are secured. Specifically, the circuit has high power capability, low output impedance and low intermodulation distortion.

The fact that this zone of advantageous screen grid tapping includes the 50% value leads to a novel and marked simplification of the circuit. In Fig. l the 50% tap 46 is obviously appropriate for connection of the screen grid 47' of tube 17 and the turns included between coil terminal 56 and tap 46 are the means by which, when tube 17 is conductive, the electron stream variation of screen grid current results in transfer of signal energy to the secondary winding 43 in addition to the energy normally transmitted through the anode circuit and both primary windings 41 and 34. That is, the one-half of primary winding 41 comprehended between terminal 56 and tap 46 constitutes the load on screen In the case of screen grid 48', primary winding 34 cannot be tapped because its direct-current potential is unsuitable for screen grid energization. But the directcurrent potential of primary winding 41 is correct for this purpose, and the alternating current potential and phase of the left end, between terminal 53 and tap 46, are correct. Therefore screen grid 48 also can be and is connected to the tap 46. Thus one-half of primary winding 41 serves as the load for one screen grid and the other half of the same winding 41 serves as the load for the other screen grid.

The graphs of Fig. 3 apply only to the specified tube and similar tubes. Other tubes may have widely different characteristics and the mentioned range of screen grid tap turn ratios of 40% to 50% may not apply. For example, the optimum tap point may vary, for tubes of various types, from 22 /z% to 55% of the total number of primary coil turns, measured from the cathode end of the coil.

When a turn ratio other than 50% is required it may be secured in various ways. Perhaps the simplest is illustrated in Fig. 4. If the desirable ratio is p percent from the cathode end, the tap 66 for screen grid 47 is placed p percent of the turns from terminal 56 and a second tap 67 is placed p percent of the turns from terminal 53 for screen grid 48.

Instead of tapping Winding 41 a closely coupled tertiary winding may be employed connected in any one of several difierent ways so that its action is magnetically similar to that shown in Fig. 4. One such circuit is shown in Fig. 5 which also shows common battery. A tertiary winding 68 of transformer 36 has a center tap 71 coupled through a large capacitor 69 to either one of the primary windings, here shown as primary winding 41. The center tap 71 of tertiary winding 68 is also connected through a voltage dropping resistor 72 to any suitable source of positive potential such as, for example, terminal 73 serving both primary windings as well, in order to energize the screen grids with direct current. The tertiary Winding can be small, its turns on each side equalling the difierence between the required screen grid tap turns and one-half of the primary turns. This circuit permits the screen grids to have a direct potential lower than that of the anodes.

What is claimed is:

1. An amplifier comprising, a pair of similar electronic tubes each having at least anode, cathode, control grid and screen grid electrodes, means for driving said pair of tubes in opposite phases, an output transformer having at least first and second primary windings and a secondary winding, each primary winding having first and second end terminals, the relative polarizations of said first end terminals being the same, the cathode of one of said tubes being connected to the first end terminal of said first primary winding, the second end terminal thereof being connected to a reference direct current potential terminal, the anode of said tube being connected to a direct current potential terminal which positive with respect to said reference potential terminal, the anode of the other of said tubes being connected to the first termi nal of said second primary winding, the second terminal thereof being connected to a positive terminal with the cathode of said other tube being connected to a relatively negative potential terminal, a conductive connection between said reference potential terminal and said relatively negative potential terminal, alternating current circuit means connecting said primary windings in parallel direct-current circuits connecting the screen grid of each of said tubes to said second primary winding of the output transformer, and a load connected to said secondary winding.

2. An amplifier in accordance with claim 1 in which said screen grids of said one tube and the other tube are both connected to a center tape of said second primary winding.

3. An amplifier in accordancce with claim 1 in which the screen grid of said one tube has a direct-current connection to a tap of said second primary winding a selected number of turns distant from the first terminal thereof and the screen grid of said other tube has a direct-current connection to another tap of said second primary winding the same selected number of turns distant from the second terminal thereof.

-4. An amplifier in accordance with claim 1 in which said alternating current circuit means includes a first capacitor interconnecting the first terminals of said first and second primary windings and a second capacitor interconnecting the second terminals of said first and second primary windings.

References Cited in the file of this patent OTHER REFERENCES Publication: Audio Engineering, May 1951, pages 15, 46, 47, 48, A Survey of Audio-Frequency Power-Amplifier Circuits, by Peter G. Sulzer. 

